System for Heating a Primary Air Stream

ABSTRACT

A system for heating a primary air stream in a steam generating process comprising at least one primary air heat exchanger to exchange heat between the primary air stream in the primary air heat exchanger(s) and a process fluid.

This invention relates to a system for heating a primary air stream in a steam generating process, generally involving a steam generator such as a coal-fired boiler, which could form part of a power plant using steam driven turbine generators and a condensate and feed heating system which extracts steam from turbine inter-stages to heat feedwater for achieving a higher efficiency, and a method for same.

In a steam generating process, there are generally one or more steam generators, one example of which is a boiler, which can be fired by various types of fuels, examples of which are coal and biomass. Such boilers for steam generation are well known, and can form part of a power plant using the steam to drive one or more steam turbine generators.

A steam generating process generally involves the supply of a number of air streams thereto and therearound for various purposes, some of which air streams are typically heated prior to use so as to ensure best efficiency of their use, usually by close or closer temperature-matching them to their place of use.

Typically, a steam generating process includes a ‘primary air stream’ for a major purpose, generally being for use with fuel preparation and transportation. Conventionally, such a primary air stream is heated by heat exchange with a hot flue gas stream in a gas-to-air heat exchanger such as a regenerative gas air heater. However, the leakage of primary air from a regenerative gas air heater can be as high as 30% due to the high pressure nature of the primary air flows therein. Furthermore, the temperature of hot flue gas streams conventionally used in such air heaters is often much higher than required particularly when fuel moisture is low. A tempering air source is therefore required to lower the temperature of the primary air stream after the heat exchange, which is not efficient. Tempering air which bypasses the air heaters may also cause high backend gas temperatures.

It is an object of the present invention to obviate one or more of the above disadvantages and to provide a better system for heating a primary air stream in a steam generating process.

According to one aspect of the present invention, there is provided a system for heating a primary air stream in a steam generating process comprising at least one primary air heat exchanger to exchange heat between the primary air steam in the primary air heat exchanger(s) and a process fluid.

The process fluid may for example be water and/or steam. The process fluid may be water and/or steam from a steam generator of the steam generating process. The process fluid may for example be water from between a condenser outlet and an economiser outlet of a steam generator of the steam generating process.

By heating the primary air stream with a process fluid such as water and/or steam from a steam generating process rather than a hot flue gas stream, the problem of primary air leakage from a regenerative gas air heater is avoided, and much greater control of the heat exchange can be achieved.

The term “primary air stream” as used herein may comprise one or more primary air streams being in series, parallel or both. Embodiments of the present invention as described hereinafter may apply to a single primary air stream, each of a plurality of primary air streams, or be variable across a plurality of primary air streams. Thus, whilst the present invention is described hereinafter in relation to a single primary air stream, the invention is not limited thereto.

The primary air stream may comprise ambient air, recycle gas, or any combination or ratio extending from 0-100% thereof, optionally with the addition of one or more further components such as a near or pure oxygen stream. The requirement of the primary air stream is to at least partly assist the preparation and/or transportation of fuel into the steam generator, optionally as well as combustion support.

Where the primary air stream comprises two or more primary air streams, each primary air stream may comprise the same or different characteristics and/or composition, including but not limited to flow rate, flow volume, temperature, pressure, oxygen content and recycle gas content.

Where the primary air stream comprises two or more primary air streams, each primary air stream may be heated the same or differently, and by the same of different number of primary heat exchangers.

Where the primary air stream comprises two or more primary air streams, two or more of such air streams may be combined after heating according to the present invention either prior to, during or after their intended use or destination.

Where the primary air stream comprises two or more primary air streams, the primary air streams may be passed separately and/or in any combination to the steam generator.

In one embodiment of the present invention, there are provided a plurality of primary air streams, each primary air stream for the preparation and/or transportation of a separate fuel stream into the steam generator, such as an equivalent plurality of fuel pulverisers, each passing a separate fuel stream into a boiler. In this way, there can be separate control of each primary air stream, providing greater control flexibility of the overall fuel preparation, transportation and supply into the steam generator.

The system may comprise two or more primary heat exchangers in series, parallel or both, providing heat exchange with one or more primary air streams, also being in series, parallel or both, such that they system may comprise any number of primary air streams and primary air heat exchangers in any combination thereof.

In one embodiment of the present invention, the primary air heat exchanger(s) comprise at least one high pressure heat exchanger and at least one low pressure heat exchanger. The terms “high pressure” and “low pressure” are known to the person skilled in the art in relation to steam generating processes, especially involving a steam generator such as a boiler and a feed heating/heat recovery system.

The process fluid may be provided in one or more streams. A plurality of process fluid streams may be provided in series, parallel or both, and optionally from a single source or a plurality of sources. Further optionally, the process stream may be provided in one or more process fluid circuits, optionally a plurality of separate circuits, being separate or connected, each circuit optionally passing through a separate primary air heat exchanger to exchange heat between the primary air stream and a process fluid stream.

The process fluid may be a process liquid. The process liquid may be any liquid or combination of liquids useable for heat exchange, including water, ammonia, alcohols, hydrocarbons and the like. Preferably, the process liquid is wholly or substantially water, optionally including one or more additives or other minor components known in the art.

The process stream may be heated (to subsequently provide heat to the primary air stream in the primary air heat exchanger(s)) by any direct or indirect process, device, unit or apparatus. This can include its direct heating by one or more boilers or the like, or its heat exchange with a hotter fluid stream such as a hotter exhaust or flue gas stream.

As the flow of a process fluid envisaged for use in accordance with the invention such as a process liquid can be controlled easily, the system of the present invention can provide better control of the heating of the primary air stream. No tempering air source supply into the primary air stream may be required to achieve the correct temperature of the primary air stream prior to its use in the steam generating process. Preferably, no primary air stream gas-to-air heaters are required.

In a possible embodiment of the present invention, the process fluid is water and/or steam from a steam generator or boiler of a steam generating process. The process fluid may for example be water tapped at a point downstream of a condensate pump of the steam generating process.

In a possible embodiment of the present invention, the process liquid is feedwater for a steam generator. Such a steam generator may be a boiler, optionally comprising one or more boilers, and optionally including an integral steam generator economiser known in the art.

Such feedwater may be provided directly or indirectly from a feedwater stream to be processed by one or more steam generators of the steam generating process involved in the present invention. Preferably, a portion of such a feedwater stream is provided as the process liquid for the system of the present invention. Such a portion may be provided as the full feedwater stream, or preferably as a slip stream of such a feedwater stream, such a slip stream generally being a minor portion of the full feedwater stream.

According to another embodiment of the present invention, the feedwater is provided from the feedwater stream in the steam generating process between the steam condenser and the exit of the steam generator economiser.

In another embodiment of the present invention, the primary air heat exchanger(s) comprise at least one high pressure heat exchanger and at least one low pressure heat exchanger. The terms “high pressure” and “low pressure” are known as subject to the pressure downstream or upstream of the feed pump(s) respectively.

For a coal-fired steam generator or boiler, the coal is typically pulverised in one or more mills prior to its use in the boiler.

According to a further embodiment of the present invention, the primary air stream passes to one or more pulverisers after being heated by the primary air heat exchanger(s), typically to two or more pulverisers.

According to another embodiment of the present invention, the primary air stream may be preheated (prior to the main system for heating the primary air stream) by at least one separate process fluid preheat exchanger, the heat for which may be provided by any suitable source such as through heat exchange with one or more exhaust streams or flue gas streams, optionally a steam generator flue gas stream.

According to a second aspect of the present invention, there is provided a steam generation system comprising a steam generator such as a boiler and a system for heating a primary air stream as hereinbefore defined. The steam generation system preferably involves one or more steam generating processes as herein described. In the steam generation system, the heated primary air stream is preferably provided to one or more pulverisers of a solid fuel such as coal or biomass for use in the steam generation.

According to a third aspect of the present invention, there is provided a method of heating a primary air steam in a steam generating process comprising at least the step of:

passing a process fluid through one or more primary air heat exchangers to exchange heat from the process liquid to the primary air steam in the primary air heat exchanger(s).

Preferably, the method of heating a primary air stream according to the present invention comprises heating the primary air stream using a system as hereinbefore defined.

The present invention encompasses all combinations of various embodiments or aspects of the invention described herein. It is understood that any and all embodiments of the present invention may be taken in conjunction with any other embodiment to describe additional embodiments of the present invention. Furthermore, any elements of an embodiment may be combined with any and all other elements of any other embodiment to describe additional embodiments.

Embodiments of the present invention will now be described by way of example only, and with reference to the accompanying drawings in which;

FIG. 1 comprises a first scheme for a system for heating a primary air stream according to one embodiment of the present invention;

FIG. 2 comprises a second scheme for a system for heating a primary air stream according to a second embodiment of the present invention; and

FIG. 3 comprises a third scheme for a system for heating a primary air stream and other embodiments of the present invention;

For the purpose of this description, a single reference number will be assigned to a line as well as a stream carried in that line.

Referring to the drawings, FIG. 1 shows a first scheme A for a system for heating a primary air stream 2 in a steam generating process. For the first scheme A, the steam generating process includes a steam generator being a boiler 4. The boiler 4 may comprise a number of inlets and outlets for the passage of a number of streams, in particular feedwater thereinto and steam therefrom. The majority of these inlets and outlets and streams are not shown in FIG. 1 for clarity purposes.

The fuel source for the boiler 4 in FIG. 1 is coal supplied as a coal stream 6. The coal stream 6 is typically pulverised in one or more pulverisers 8 so as to be provided to the boiler 4 as a pulverised coal stream 10.

The use of the primary air stream 2 in the first scheme A of FIG. 1 is as an air stream into the one or more pulverisers 8. For achieving the drying capacity in the pulverising process, it is typically desired to provide the primary air stream thereto at a raised temperature, such as above 150° C. or above 200° C.

The first scheme A of FIG. 1 shows at least one primary air heat exchanger 12 able to exchange heat between the primary air stream 2 and a process liquid. The process liquid is provided as a process liquid stream 14 through an inlet (not shown) into the primary air heat exchanger 12, to provide a heated primary air stream 2 a and a cooler process liquid stream 14 a. The first scheme A of FIG. 1 also shows a flue gas stream 16 from the boiler 4.

FIG. 2 shows a second scheme for heating a primary air stream according to a second embodiment of the present invention. In the scheme of FIG. 2, primary air can be provided having ambient properties and conditions, and/or be partly or substantially recycle gas (derived from the flue gas), and this typically passes through a primary air fan (not shown) to increase its pressure and provide an initial primary air stream 22.

The initial primary air stream 22 can be preheated by at least one separate process liquid preheat exchanger 24. A process liquid preheat stream 26 for providing the heat to the initial primary air stream 22 can be directly or indirectly heated by any hotter stream, unit, device or apparatus. An example of the heating of the process fluid preheat stream 26 is through heat exchange in a first flue gas heat exchanger 28, such that the process fluid preheat stream 26 is connected with a first flue gas exchange stream 30 passing through the first flue gas heat exchanger 28. Optionally, the path of the process fluid preheat stream 26 passes through one or more other heat exchangers or other units or devices.

The preheat exchanger 24 increases the temperature of the primary air stream 22, such as from an ambient temperature, which could be about 20-25° C., to a higher temperature, such as >50° C., or below, near or above 100° C., to provide a preheated primary air stream 22 a.

The second scheme of FIG. 2 shows the preheated primary air stream 22 a being split at a junction 27 into two equal or non-equal part streams 22 b, 22 c. Any suitable divider or splitter could be used at the junction 27 to create the part streams, and optionally there could be created one or more further part streams illustrated in FIG. 2 as stream 22 d.

In the path of the part streams 22 b, 22 c, FIG. 2 shows two primary air heat exchangers, being first low pressure heat exchangers 32 a, 32 b respectively, and in-line subsequent high pressure heat exchangers 34 a, 34 b respectively.

The low pressure heat exchangers 32 a, 32 b are provided with process liquid as first process liquid streams 36 a, 36 b, said streams being the same or different in terms of any characteristic, composition or property, such as flow rate, volume, temperature, pressure, etc. The process liquid of the first process liquid streams 36 a, 36 b is preferably one or more slip streams of the main feedwater stream to be subsequently processed by the steam generator of the steam generating process. In FIG. 2, the steam generator is shown as a coal-fired boiler 40.

The first process liquid streams 36 a,b are able to provide heat to the preheated primary air streams 22 b,c entering the low pressure heat exchangers 32 a,b through inlet to provide hotter primary air streams 22 d, 22 e, which hotter streams 22 d,e then pass to inlets of the high pressure primary air heater exchangers 34 a,b. The low pressure heat exchangers 32 a,b may increase the temperature of one or both of the preheated primary air streams 22 b,c by another 50° C. or 100° C. or more, said increases being the same or different, whilst the first process liquid streams 36 a,b are provided through outlet of the low pressure heat exchangers 32 a,b as cooler first process liquid streams 42 a, 42 b.

The high pressure primary air heat exchangers 34 a,b are provided with a process liquid as second process liquid streams 44 a, 44 b through suitable inlets. The second process liquid streams 44 a,b may again be the same or different in terms of any characteristic, composition or property, such as flow rate, volume, temperature, pressure, etc. Said streams may also be provided from the same or a similar source as the first process liquid streams 36 a,b.

Preferably, the second process liquid streams 44 a,b are a portion, such as a slipstream, of the main feedwater stream to be subsequently processed by the steam generator of the steam generating process. Optionally, the second process liquid streams 44 a,b are provided from the cooler first process liquid streams 42 a,b, typically following further processing of the cooler first process liquid streams 42 a,b, in particular increasing its pressure and/or temperature.

The second process liquid streams 44 a,b provide heat to the hotter primary air streams 22 d,e so as to provide two final primary air streams 22 f, 22 g from outlets of the high pressure heat exchangers 34 a,b respectively. The final primary air streams 22 f,g could have a temperature greater than 200° C.; not being limited thereto.

Each final primary air stream 22 f,g can then provide a primary air stream into two separate pulverisers 46 a, 46 b being provided by two coal streams 48. The pulverisers 46 a,b provide two pulverised coal streams 50 into the boiler 40 in a manner known in the art.

With separate control of the heating of each part stream 22 b,c, as well as any other such part streams being directed to one or more further pulverisers, greater flexibility can be provided to the overall provision of the fuel and its transport stream into the boiler 40, in particular the pulveriser exit temperature of each coal stream 50. Typically there are 4-10 pulverisers, such that the system of the present invention can provided very close control over each fuel line into the boiler 40 to maximise its efficiency as other parameters change, such as the overall steam generator demand.

The high pressure primary air heat exchangers 34 a,b provide cooler second process fluid streams 52 a,b which can then be directly or indirectly passed into the boiler 40 for use as the boiler feedwater.

The cooler first and second process fluid streams 42 a,b and 52 a,b may be combined with one or more other feedwater streams 54, such as the majority or main feedwater stream remaining after the provision of any slip stream(s) providing the process fluid streams 36 a,b and 44 a,b, to provide a fuller feedwater stream 56 into the boiler 40. FIG. 2 shows two illustrative incoming streams 57 representing possible returning feedwater slip streams.

The boiler 40 generates steam which could be used in a number of processes, including but not limited to steam driven turbine generators. The boiler 40 also produces an exhaust flue gas stream 60. The flue gas stream 60 can be provided to one or more heat exchangers known in the art. Any cooler flue gas stream 66 may still have sufficient residual heat to heat a further stream in a heat exchanger 28, such as the process fluid preheat stream 26 as an exchanger stream 30.

The first and second schemes shown in FIGS. 1 and 2 may include further apparatus, device, units, streams, conduits and the like to complete a steam generating process, which further elements are not shown for clarity. For example, FIG. 3 shows flue gas 60 being split at a first junction 61 to pass through a secondary air stream heat exchanger 62 to heat a secondary air stream 63, and through one or more other heat exchangers 64 such as one or more process liquid economisers, prior to recombining at a second junction 65. The cooler flue gas stream 66 from the second junction 65 passes through an electrostatic precipitator (ESP) 82 and an induction fan 84, followed by passage via a suitable inlet into a downstream process fluid heat exchanger 86. The downstream process fluid heat exchanger 86 is able to extract any remaining available heat energy from the flue gas with a downstream process fluid in a first process circuit 100.

The first process circuit 100 comprises a process fluid in at least a first downstream process fluid conduit 102 passing via a suitable inlet into the downstream process fluid heat exchanger 86 to provide, via a suitable outlet, a hotter downstream process fluid stream 104 in a second fluid conduit 104. At a first circuit junction 106, the hotter downstream process fluid stream 104 may be divided by a suitable controller anywhere between 0-100% between a secondary air heating stream 108 and a primary air heating stream 110.

The secondary air heating stream 108 passes via a suitable inlet into a secondary air preheat exchanger 112 to provide heat exchange with an initial secondary air stream 90, so as to provide some pre-heating to the secondary air prior to the subsequent heating of the secondary air stream 63 by a regenerative gas secondary air heater 62.

Similarly, the primary air heating stream 110 can provide some pre-heating to an initial primary air stream 92 (after passage through a primary air fan 94) in a primary air preheat exchanger 114, to provide a primary air stream 70 to be subsequently heated by one or more primary air heat exchangers 72 by a process liquid stream 74 in accordance with the system of the present invention to provide a heated primary air stream 76, which passes into one or more pulverisers 80 as described above.

The downstream process fluid of the downstream process fluid circuit 100 may be any suitable liquid, gas or combination of same, usually at low pressure and usually circulated by one or more suitable circulation pumps in the circuit 100.

The primary and secondary air preheat exchangers 112, 114 provide cooler return streams 116, 118 respectively, which can be recombined to provide the process fluid in the first downstream process fluid conduit 102. The downstream process fluid heat exchanger 86 provides a cooler flue gas stream 88.

The initial primary air stream 92 and secondary air stream 90 can be provided from a single air source stream 94, prior to being divided by a suitable controller. The source of stream 94 may be ambient air, recycle gas or any combination thereof, the recycle gas optionally being at least partly provided by the cooler flue gas stream 88, optionally after further processing thereof.

In a first alternative, the hotter downstream process fluid stream 104 in the second fluid conduit 104 passes through a heat exchanger on the single air source stream 94 to heat the primary air and secondary air prior to their division.

The downstream process fluid circuit 100 increases the use of the available heat energy in the flue gas to provide some pre-heating of the primary and secondary air streams so as to maximise the efficiency of the steam generation system shown in FIG. 3.

The schemes of FIGS. 1-3 show the heating of a primary air stream by the use of a process liquid, optionally being provided in one or more streams, and optionally with one or more separate process liquid preheat streams. In this way, the system of the present invention is able to avoid the use of any primary air heat exchangers being gas-to-air heaters, in particularly regenerative air heaters, which can leak as much as 30% of the primary air stream at the air heater inlet due to high pressure of the primary air stream. Further, any tempering air source or tempering air system can be avoided, as the primary air stream temperature can be controlled accurately by the use of process liquid(s).

Furthermore, process liquids, especially water, are used in the schemes shown in FIGS. 1-3 to provide the heat to all the heat exchangers for the primary air stream. Water provides significant advantages over steam by being able to be transported, such as around a circuit, with minimal pressure loss over distance, even using small pipes. By carefully selecting and controlling the water flow, and the temperature differentials in the primary air heat exchangers, there can be optimising of the heat exchange from a process liquid such as water to a primary air stream.

In addition, as the specific heat capacity of water is much higher than the specific heat capacity of gases such as air, the flow of the process liquid such as water can be minimised as well as reducing the OPEX of the system.

In addition, erosion, corrosion and fouling would not be the potential problems for these process liquid PA heaters as they are not exposed to the ash borne corrosive flue gas.

In the example embodiments use is made of boiler feed water from the condenser outlet to the economiser outlet as the heat source. The primary air heaters are water PA heaters.

If the system is considered in conjunction with a once through boiler with a no pump start up system, the economiser outlet water temperature can be much lower than saturation temperature during boiler start up. It is possible to consider as a heat source not just water up to economiser outlet but a point from the boiler in general including both water and steam. The primary air heaters are then water/steam PA heaters.

Another interesting advantage may arise in respect of early service of the coal milling system. The boiler is normally started with oil to raise boiler load to a certain level so that the flue gas temperature is high enough to heat primary air to a sufficient temperature for the drying process of the milling system. With water/steam PA heaters, the required primary air temperature can be achieved at a lower load compared to the conventional design where primary air comes from gas/air heaters. With the earlier service of the milling system, pulverised coal can be fired instead of oil with lower boiler output so that the oil consumption can be much reduced. With oil prices considerably higher than coal, this is a significant advantage where boilers are started and stopped frequently. 

1. A system for heating a primary air stream in a steam generating process comprising at least one primary air heat exchanger to exchange heat between the primary air stream in the primary air heat exchanger(s) and a process fluid.
 2. A system in accordance with claim 1 comprising two or more primary air heat exchangers.
 3. A system in accordance with claim 2 wherein the primary air heat exchangers comprise at least one high pressure heat exchanger and at least one low pressure heat exchanger.
 4. A system in accordance with any one of the preceding claims wherein the process fluid comprises two or more streams.
 5. A system in accordance with claim 4 wherein each process fluid stream passes through a separate primary air heat exchanger.
 6. A system in accordance with any one of the preceding claims wherein there is provided no tempering air source supply into the primary air stream.
 7. A system in accordance with any one of the preceding claims wherein the primary air stream comprises two or more primary air streams, each primary air stream comprising the same or different characteristics, properties and/or composition, including but not limited to flow rate, flow volume, temperature, pressure, oxygen content and recycle gas content.
 8. A system in accordance with claim 7 wherein each primary air stream passes to a respective pulveriser.
 9. A system in accordance with claim 7 or claim 8 wherein each primary air stream is separately heated by separate primary air heat exchanger(s).
 10. A system in accordance with any one of the preceding claims wherein the process fluid is water and/or steam from a boiler of the steam generating process.
 11. A system in accordance with any one of the preceding claims wherein the process fluid is a process liquid.
 12. A system in accordance with claim 11 wherein the process liquid is water.
 13. A system in accordance with claim 12 wherein the process liquid is feedwater for a steam generator.
 14. A system in accordance with claim 13 wherein the process liquid is a portion of a feedwater stream to be processed by a steam generator of the steam generating process.
 15. A system in accordance with claim 14 wherein the process liquid is a slipstream of the feedwater stream to be processed by the steam generator of the steam generating process.
 16. A system in accordance with any one of claims 13 to 15 wherein the feedwater is provided from the feedwater stream in the steam generating process between the steam condenser and the exit of the steam generator economiser.
 17. A system in accordance with any one of the preceding claims wherein the primary air stream passes to one or more pulverisers after being heated by the primary air heat exchanger(s).
 18. A system in accordance with any one of the preceding claims wherein none of the primary air heat exchanger(s) are gas-to-air heaters.
 19. A system in accordance with any one of the preceding claims wherein the primary air stream is preheated by at least one separate process fluid preheat exchanger.
 20. A steam generation system comprising a steam generator such as a boiler and a system for heating a primary air stream as defined in any one of claims 1 to
 19. 21. A method of heating a primary air stream in a steam generating process comprising at least the step of: passing a process fluid through one or more primary air heat exchangers to exchange heat from the process liquid to the primary air steam in the primary air heat exchanger(s).
 22. A method in accordance with claim 21 comprising heating the primary air stream using a system as defined in any one of claims 1 to
 17. 